1. Field of Invention
This invention pertains to closed-loop regulation of infusion of fluids, medications and diagnostic substances. More particularly, it pertains to such infusion regulation by continuous measurement of urine production and composition as an indication of the physiological condition of a patient.
2. Prior Art
The medical profession has long been familiar with the value of urinalysis as an index of the physiological state of a patient, particularly with regard to hematology, metabolism, renal function and the body waste systems. Typically, urinalysis is conducted by obtaining a urine sample and processing it through a series of laboratory tests which are primarily directed to identifying the nature of urine constituents as opposed to urine volume. Although this system has proved adequate for periodic urinalysis, numerous medical conditions require a more constant evaluation of urine output, including production rates and variation.
One such field of primary significance includes medical treatments for burns and major trauma. In addition to monitoring urine constituents, the mangement of fluid resuscitation is of critical importance. In the past, substantial empirical judgment has been necessary in structuring clinical programs to maintain a patient at proper fluid volumes where substantial portions of his body has been burned or subjected to other major trauma. Obviously, fluid loss in such circumstances is serious and must be remedied and regulated to insure proper recovery.
Historically, a major cause of death from burns was the tremendous fluid shifts that occur immediately post-burn. Many studies have confirmed the importance of sodium-containing crystalloids in volumes up to 15-20% of total body weight as the imperative resuscitation regime following thermal injury. Authors of such studies have emphasized the maintenance of urine output of approximately 30-50 cc/hr. as the most reliable parameter in assessing the adequacy of fluid therapy. Similar studies have led to parallel emphasis on vigorous fluid resuscitation in all forms of major trauma and shock. Although sophisticated invasive techniques have been developed to monitor hemodynamic status and cellular metabolism in such shock, trauma and burn patients, maintenance of adequate urine output remains an important parameter in medical treatment.
This significant role of urine output is based on numerous considerations. For example, the kidney is one of the three vital organs that must be protected by intravascular volume repletion. Also, the maintenance of urine flow at a brisk level tends to protect against the development of acute tubular necrosis and assist in wash-out of myoglobin and hemoglobin from renal tubules. Each of these treatment aspects is especially important to the burn and trauma patient. Furthermore, urine output is a valuable indicator of adequate fluid balance within the patient. Although urine output may correlate rather poorly with fluid administration during inappropriate diuresis and nonoliguric renal failure, the maintenance of urine output above 30 millimeters per hour is strongly suggestive that at least one part of the body is being profused appropriately.
To the physician equipped to monitor cardiac output, mixed venous oxygen tension, etc., urine flow provides confirming evidence of the organism's response to physiologic manipulations and is especially important in vasopressor therapy. This supplemental input from urine flow is even more significant in burn patients since cardiac output is universally depressed with burns and other forms of trauma. Moreover, the cardiovascular response to many types of trauma is still poorly understood. Therefore, in such circumstances, urine output provides a familiar reassurance of the meaning of these more complex parameters. In addition to these technical considerations in support of the importance of urine output as a medical parameter, it is important to note that in a patient having a catheter in place, urine output is an accessible parameter for measurement which does not involve additional invasive procedures which risk infection and add to patient discomfort.
Despite the accessibility and clinical utility for measurement of urine output, however, the medical industry still places primary reliance on manual volumetric measurements of urine flow to control and regulate fluid resuscitation. Some treatment methods still rely on burn fluid formulas which provide a calculated fluid infusion rate based on body weight, extent of burned area, etc. Although such formulas may be useful in developing an initial infusion rate, the broad disparity between actual patient response from the norm, along with the practical limitation of having to manually adjust fluid infusion rates impairs proper manual fluid administration. For example, nurses with many other important duties find it difficult to record frequent urine output, attend to catheter and collection systems, adjust IV's, etc. Too often, the result is confusion as to patient response to fluids, inaccurate recording of outputs and a tendency to over-resuscitate the trauma or burn patient. Because of these practical limitations, little progress has been made in understanding the physiologic balance between fluid therapy and urine output. Although sophisticated bench work has broadened an understanding of kidney function at the cellular level, the translation of such information into treatment methods for the critically ill patient leaves much to be desired.
Initial in-roads into measurement of urine output have been accomplished by the University of Alabama, University of Washington, and Roche, Inc. using instruments which periodically weigh the excreted urine. Other investigators have used ultrasonic measuring devices and volumetric devices; however, such systems tend to be very bulky and to pose contamination and handling problems.
Some progress in computer controlled infusion systems has been realized by the University of Washington and University of Alabama. The Washington system uses a small microprocessor to monitor mean arterial pressure, central venous pressure and hourly urine output. The IV infusion rate is claimed to be automatically controlled to maintain blood pressure and urine output within acceptable limits. Unfortunately, this system requires vascular invasion for arterial and venous monitoring. In addition, the maintenance of consistent blood pressure as a goal of fluid infusion may prove difficult to adapt to a clinical application. This is because the blood pressure of an awake, traumatized man is subject to a myriad of regulating forces including vascular volume, peripheral resistance, catecholamine and other hormonal flux, plus the major contributions of anxiety, pain and level of consciousness. It is questionable, therefore, whether any fluid regime can successfully maintain blood pressure within a narrow range for such patients.
The University of Alabama on the other hand, uses left atrial pressure to determine the volume of fluid to infuse. The infusion rate is modulated by blood pressure parameters and periodic (1 hour) weighing of urine output. Although some success has been noted with this procedure, this method also necessitates invasive monitoring and is subject to the effects of other heart functions such as cardiac output, stroke work index, etc.